Src: fix GCC compiler error
Src: fix GCC compiler error

The compilation failed with
 gcc (GCC) 4.8.3 20140911 (Red Hat 4.8.3-9) on Amazon Linux AMI,
 (Raspbian 4.9.2-10) 4.9.2.
The compilation suceeded before already with
 Apple LLVM version 7.3.0 (clang-703.0.31) on OS X.

deepstreamIO/deepstream.io-client-cpp,deepstreamIO/deepstream.io-client-cpp,deepstreamIO/deepstream.io-client-cpp,deepstreamIO/deepstream.io-client-cpp